For instrument applications such as universal counters and oscilloscopes, a peak detector for repetitive waveforms is a valuable addition to the various input channels. It can provide a means for automatic triggering, signal amplitude measurement, and other waveform parameter measurements such as risetime. To be adequate for this application, the peak detector must exhibit good accuracy over a wide input dynamic range and over the instrument's entire operational frequency range. Furthermore, it must also exhibit good accuracy with any arbitrary input waveshape. Good accuracy in this case means the output detected voltage must be a DC level within a few millivolts of the input peak level.
Peak detectors in the prior art cover a variety of circuits with different operational characteristics. For example, there are detectors for amplitude modulation (AM), that is, envelope detectors, and detectors for detecting and holding the amplitude of a single pulse indefinitely. Input waveforms and the corresponding detected output waveforms for these prior art detectors are illustrated in FIGS. 1A and 1B.
The AM detector for the waveform in FIG. 1A is common and simple; it typically comprises only three elements: a diode for a unidirectional flow of current; a capacitor for storing the peak amplitude; and a resistor for discharging the capacitor. This circuit is shown in FIG. 1D. This type of detector is normally designed for use only with one carrier frequency where its output must decay fast enough to follow the lower modulating frequency to develop the modulating envelope. Further, the input waveshape for this type of detector is known and is usually a sinusoid of fixed frequency. Because of the diode in series with the input, there is always a difference of approximately 0.6 volt between the input and output voltages caused by the forward voltage drop of the diode. But, since only the envelope information is desired, the fact that at the peaks the input differs from the output by this forward voltage drop of the diode ([V.sub.F ].sub.pk) has no significance. This would not be the case if this type of detector is used for wideband applications. There, the presence of the forward voltage drop of the diode, the fast decay for the circuit to function properly at low frequencies, and the dependence on input waveshape would make this type of detector unsuitable for wideband applications.
The detector used for short pulses is generally designed to achieve an accurate peak output voltage reflecting the peak input voltage. Further, it is designed to hold the peak voltage for a relatively long time. A typical circuit of this type of detector is shown in FIG. 1C. This type of circuit generally has no inherent output decay to allow the output to follow a slowly decreasing input amplitude. This can be seen in the input and output waveforms in FIG. 1B. Although nominally wideband in nature, because it detects pulses, the upper frequency limit to these circuits is only a few megahertz. Therefore, this type of detector is also not suitable for wideband, repetitive waveform detection. It does, however, have the accuracy lacking in the detectors, because of the typical inclusion of the diode in the voltage follower feedback loop. One can take advantage of this fact and add an element, e.g., a resistor or a current sink, to slowly discharge the storage capacitor to provide the desirable output decay. To extend the circuit to high frequency performance beyond a few megahertz, however, requires a high gain feedback amplifier with extreme bandwidth and stability beyond the input frequency range. This is extremely difficult and costly to achieve.